Recent advances in microCT machines have made it possible to image with a resolution of <10 m. Scans take ~6 to 20 minutes to acquire, and current iodine contrast agents due to their short half life are not generally suitable. We recently commercialized gold X-ray contrast agent, AuroVist, but they are more expensive than iodine agents. We now propose to develop heavy metal nanoparticle contrast agents based on tungsten and bismuth that are lot cheaper than our gold-based contrast agents and more absorptive than iodine. The new agents can be functionalized with reactive groups for covalent cross-linking to targeting molecules and will enable in vivo visualization of organ function, angiogenesis, repair, tumor formation, metastasis, studies of response to drugs and other therapies or conditions, such as ischemia or radiation, and basic biological studies of animals including development, infection, immune response, cardiac studies, tissue injury and healing. The iodinated contrast agents cause contrast induced reactions (CIRs) in many patients, including those with allergies, asthma, kidney diseases and diabetes. For patients with compromised kidneys the CIRs can have serious consequences. The CIRs are attributed to the high viscosity and osmolality of the iodine agents. The proposed novel surface modifications to the heavy metal agents will improve their viscosities. The smaller tungsten and bismuth nanoparticles with < 5.5 nm hydrodynamic diameter can hold >250 metal atoms per particles as compared to 3 or 6 iodine atoms; they will have much lower osmolality and produce higher contrast, and help alleviate problems related to contrast induced nephropathy. Finally, because the K-edges of tungsten (69.52 keV) and bismuth (90.5 keV) are well separated from iodine (33.12 keV), simultaneous double contrasting (K-edge imaging with iodine and tungsten/or bismuth) will be possible with dual-energy (dual-beam/dual-detector) CT. PUBLIC HEALTH RELEVANCE: The proposed novel non-toxic tungsten and bismuth contrast agents for in vivo microCT X-ray imaging have higher attenuation coefficients than iodine agents and will reduce X-ray dose to millions of patents, and when used with iodine agents will enable simultaneous imaging of two targets with dual-energy CT.